Hydrostatic transmission

ABSTRACT

A hydrostatic transmission design incorporating a filter housing mounted adjacent to check valves in a center section. The filter housing can be secured to the center section by the check valves, which also may include high pressure bleeds to allow hydraulic fluid to flow from the hydraulic circuit. A shield may be located between the high pressure bleeds and the filter to prevent damage by the fluid to the filter as it exits the bleeds. The filter housing can also include integral projections on which a bypass actuation member can be mounted.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/196,183 filed Nov. 20, 1998, now U.S. Pat. No. 6,185,936.

BACKGROUND

This invention relates to an improved design of a hydrostatictransmission (“HST”) and includes several novel features. Hydrostatictransmissions are well known in the art, and are more fully describedin, e.g., U.S. Pat. No. 5,314,387, which is incorporated herein in itsentirety. Many of the inventions described herein can also be adaptedfor use in an integrated hydrostatic transmission (“IHT”) incorporatingoutput gearing and axles within a single housing.

In general, an HST has a hydraulic pump and a hydraulic motor mounted ina housing. The pump and motor are hydraulically linked through agenerally closed circuit, and both consist of a rotatable body withpistons mounted therein. Hydraulic fluid such as oil is maintained inthe closed circuit, and the HST generally has a sump or reservoir withwhich the closed circuit can exchange oil. This sump may be formed bythe housing itself.

The pump is usually driven by an external motive source such as pulleysor belts connected to an internal combustion engine. The pump pistonsengage a moveable swash plate and, as the pump is rotated by an inputsource driven by the external engine, the pistons engage the swashplate. Other HST designs may use a radial piston or ball piston pump andmotor design, but the general operation is similar, and this inventionis not limited to use with a specific design. Movement of the pumppistons creates movement of the hydraulic fluid from the pump to themotor, causing rotation thereof. The motor pistons are engaged against afixed plate, and rotation of the motor drives an output shaft engagedthereto. This output shaft may be linked to mechanical gearing andoutput axles, which may be internal to the HST housing, as in an IHT, orexternal thereto.

The pump/motor system is fully reversible in a standard HST. As theswash plate against which the pump pistons move is moved, the rotationaldirection of the motor can be changed. In addition, there is a “neutral”position where the pump pistons are not moved in an axial direction, sothat rotation of the pump does not create any movement of the hydraulicfluid.

The HST closed circuit has two sides, namely a high pressure side inwhich oil is being pumped from the pump to the motor, and a low pressureor vacuum side, in which oil is being returned from the motor to thepump. When the swash plate angle is reversed, the flow out of the pumpreverses so that the high pressure side of the circuit becomes thevacuum side and vice versa. This hydraulic circuit can be formed asporting formed within the HST housing, or internal to a center sectionon which the pump and motor are rotatably mounted, or in other waysknown in the art. Check valves are often used to draw hydraulic fluidinto the low pressure side to make up for fluid lost due to leakage, forexample. Such check valves may be located so that they directly contactthe porting or they may be located separate from the porting andconnected through additional bores to the closed circuit.

There is a need to have a means to open, or bypass, this closed circuitin certain circumstances. For example, when the vehicle is stopped, theoil in the closed circuit provides hydraulic braking, making itimpossible to manually move the vehicle. Mechanical bypass designs areknown in the art and are described in, for example, U.S. Pat. No.5,010,733. Such designs generally achieve bypass by opening the closedhydraulic circuit to the sump by, e.g., opening check valves in thecircuit, or by opening a shunt between the high pressure and lowpressure sides of the circuit. Such designs are generally complicatedand add significantly to the cost of the unit.

SUMMARY OF THE INVENTION

This invention addresses the shortcomings in prior HST bypass designs,as an improved mechanical bypass system for a hydrostatic transmissionis disclosed herein. One particular improvement of this design is in thetolerances allowed, as this design reduces or eliminates many of thetolerance issues which arise from known bypass designs. This inventionuses a filter housing secured to the bottom of the center sectionindirectly by the check plugs, and a filter secured to the filterhousing. The bypass actuator is mounted inside the filter housing and isactuated by means of a bypass rod which can extend outside the housingof the hydrostatic transmission to be operated by the user. Rotation ofthe rod causes the actuator to engage the check balls to unseat themfrom the check plug and allow the unit to enter the bypass mode. Otherembodiments include use with an HST where the hydraulic porting isintegrally formed with the transmission housing and the filter housingand filter are thus secured directly to the transmission housing.

The housing design disclosed herein is a significant improvement overcurrent transaxle designs. Using a traditional transaxle design, it isvery difficult to achieve rear discharge, as the input shaft is near thevehicle centerline. Some designs have attempted to overcome this problemby mounting the transaxle on the same deck as the engine, and usingconnecting chains to another axle on which the tires are mounted. Such adesign adds significantly to the overall cost of the unit.

One aspect of this invention is the use of a housing formed of twopieces, generally divided along a vertical axis with respect to theorientation of the output axles. One section of the housing or casing ismuch narrower than the other housing to maintain clearance between thebody of the transmission and the vehicle frame on one side, in order toaccommodate a rear discharge chute. Many of the HST elements internal tothe housing are contained in the larger of the two casing portions. Inaddition, the external housing elements are designed to conform asclosely as possible to the shape of the internal IHT elements, so as tominimize the amount of material needed and the overall size of the unit.In essence, this design allows the main housing component to be offsetto one side of the vehicle, while still maintaining the input shaft ator near the vehicle center line. Thus, the discharge chute parallels thevehicle frame, rising up slightly to clear the axle horn.

A further object of the invention is to provide an HST having animproved swash plate mounted on at least one trunnion which is securedto the transmission casing, to offer lower control moments for the swashplate. This design offers improved control of the swash plate, which isparticularly important for use of a foot control mechanism.

A further object of this invention is to provide an improved and noveldesign of a center section for an HST, whereby the output shaft of thehydrostatic motor is secured at least partially by the center sectionand is positioned so that the axis of the output shaft is located belowthe plane of the surface on which the hydrostatic pump is mounted on thecenter section. The benefits of this arrangement include, among otherthings, a reduced height of the pump, motor and center section, whichcan reduce the overall height of the unit and/or provide moreflexibility for mounting other HST elements. The horizontal mounting ofthe center section also allows for the use of the vertical split line asdisclosed herein and the unique arrangement of the HST elements withinthe housing units.

A further object of this invention is to provide an improved and novelexpansion chamber that can be bolted or otherwise secured to the HST andwhich prevents leakage or spillage of the hydraulic fluid therefrom. Ina preferred embodiment this chamber is external to the housing andincludes an internal tube extending from the top of the tank to thebottom, although variations on this design will be obvious to one ofskill in the art. The use of an external tank allows for use of asmaller transmission housing, and reduces the possibilities of leakagedue to gear splash and oil movement at various operating angles. Theinternal tube provides siphoning action which allows for, among otherthings, greater flexibility in the location of the tank.

A further feature disclosed herein in one embodiment is an improveddesign of a friction pack which enables the vehicle user to maintain theposition of the pump swash plate, and thus the speed and direction ofthe vehicle. Friction packs have been known for years in connection withHSTs and have been shown in, for example, U.S. Pat. No. 5,201,692. Theimproved design shown in the figures affords additional benefits thatwill be discussed herein.

A further embodiment of this invention provides a clip assembly securedto the ends of the axle horns to prevent excessive wear on the die casttransmission housing due to contact with the wheels. A pair of wheelsare mounted at the ends of the axles and secured thereto by means of aretaining ring or other mechanism at the end of each axle. Many vehiclemanufacturers will install washers on the axles between the wheels andthe housing in order to space and locate the wheels. During operation ofthe vehicle, the wheels or the washers, as the case may be, can beforced into contact with the die cast aluminum housing, which can resultin damage to the housing and oil seal. A clip composed of a materialsuch as spring steel can be secured at the end of the housing to providethe necessary wear surface and prevent direct contact between the diecast housing and the wheels or washers.

There is also a need in the industry for being able to review a unit andreadily determine information about the unit, such as its place and dateof manufacture or similar information. At the present time, suchinformation is generally placed on a unit by means of a label. Thiscreates additional costs in both parts and assembly, and placement of alabel on such a unit is made difficult by the obvious problem of oilpresent on the unit during the assembly process. The present inventionin one embodiment solves this problem by use of a “information pad”comprising a series of protrusions on the external housing of the unit,which may be machined or left in the natural state, to create a varietyof patterns. These patterns can be used as a code for any informationthe manufacturer may wish to include. The cost of machining for smallexternal pieces is relatively small, and once the unit is so coded, thecode will always be visible and accessible.

Other benefits and objects of this invention are disclosed herein andwill be obvious to readers of ordinary skill in the art. The featuresdisclosed herein can be combined to create a unique hydrostatictransmission design; it is understood, however, that such features areunique in their own right and can be used independently with othertransmission designs, as will be obvious to one of ordinary skill in theart.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an external housing for an integratedhydrostatic transmission in accordance with the present invention.

FIG. 2 is an exploded view of a center section and bypass mechanism inaccordance with the present invention.

FIG. 3 is a sectional side view along the lines 3—3 in FIG. 5 showing ahydrostatic transmission incorporating a center section and bypassmechanism in accordance with the present invention, and showing adifferent embodiment of a torque bracket.

FIG. 4 is a detailed sectional view of the center section and bypassunit shown in FIG. 3.

FIG. 5 is a top view of the transmission in accordance with oneembodiment of the present invention.

FIG. 6 is a cross-sectional side view of a transmission in accordancewith one embodiment of the present invention, along the lines 6—6 inFIG. 5.

FIG. 7 is the same cross-sectional side view of a transmission as shownin FIG. 6, in accordance with another embodiment of the presentinvention, showing the differential block as shown in FIG. 9.

FIG. 8 is a side sectional view along the lines 8—8 of FIG. 5.

FIG. 9 is a side view of the transmission with one portion of the casingremoved.

FIG. 10 is a side view of the transmission, with one portion of thecasing and the bevel gears of the differential removed.

FIG. 10-A is a side view of the transmission similar to that shown inFIG. 10, with a different embodiment of the differential.

FIG. 11 is a side view of the center section hydrostatic pump and motorand swash plate of the subject invention.

FIG. 12 is a perspective view of the swash plate of the subjectinvention.

FIG. 13 is another perspective view of the swash plate of the subjectinvention.

FIG. 14 is a sectional side view of the external expansion chamber ofthe subject invention.

FIG. 14-A is a sectional side view of the external expansion chamber asshown in FIG. 14, and also showing portions of the transmission housing.

FIG. 15 is a partial side view of a portion of a transmission andlocking clip incorporating an embodiment of this invention.

FIG. 16 is an end view of the transmission housing and locking clipshown in FIG. 15.

FIG. 17 is a perspective view of the center section.

FIG. 18 is an expanded view of the floating friction pack in accordancewith one embodiment of this invention.

FIG. 19 is a view of the floating friction pack of FIG. 18 mounted onthe transmission housing.

FIG. 20 is a partial sectional view of the floating friction pack asshown in FIG. 19.

FIG. 21 is a rear view of a tractor using a transmission in accordancewith one embodiment of the present invention.

FIG. 22 is a side view of the external housing, showing a secondembodiment of the return to neutral feature of the present invention.

FIG. 22-A is another side view of the external casing design.

FIG. 23 is a top view of an alternative embodiment of the externalhousing for a hydrostatic transmission, without the external controls.

FIG. 24 is a top view of another alternative embodiment of the externalhousing for a hydrostatic transmission, without the external controls.

DETAILED DESCRIPTION OF THE INVENTION

The figures herein, and in particular, FIGS. 1, 3, 5, 6 and 9 illustratean IHT configured with a vertically split housing with main casing 21and side casing 22. The arrangement of these housing elements are a keyfeature of the design, but certain embodiments of this invention do notrequire any specific housing configuration, and other housingconfigurations can be accommodated therewith. All specifics of an IHTare not shown in these figures, as the general operation of an IHT isknown in the art. In general, where different embodiments of the variouselements of the transmission are shown in different figures, likenumerals designate like elements.

Pump 11 is disposed on center section 10 and receives input shaft 24,which communicates with and is driven by a vehicle engine (not shown).Center section 10 includes internal porting 25 that hydraulicallyconnects pump 11 comprising pump cylinder block 17 and pump pistons 28and a hydraulic motor comprising cylinder block 27 and motor pistons 32.Pump pistons 28 engage adjustable swash plate 23 to create pressurewithin internal porting 25. As shown in, e.g., FIGS. 3 and 8, pistons 28generally include a spring 124 mounted therein and piston washer 125placed in the top of piston 28 to prevent damage to the piston by spring124.

Casings 21 and 22 form an internal sump or reservoir 43 external tocenter section 10. Motor cylinder block 27 is connected to and drivesoutput shaft 66, which in turn drives various reduction gears, includinggear 67, gear 69, gear 70 and differential 68 including bull gear 72.Differential 68 is in turn operatively connected to the output driveaxles 90A and 90B of the vehicle.

As shown in FIGS. 2, 11 and 17, center section 10 has a motor runningsurface 12 and a pump running surface 14, on which motor cylinder block27 and pump cylinder block 17 are respectively mounted for rotation.Center section 10 acts as, among other things, a mounting unit for thepump and motor of the hydrostatic transmission.

One goal of the invention is to minimize the effort required tomanufacture such an HST, and to minimize the number of fasteners needed.Furthermore, the use of the horizontal connections between centersection 10 and casing 21 allows for the vertical split configurationshown herein, with most of the HST elements being located in main casingsection 21.

As shown in FIGS. 2 and 17 center section 10 can be secured to maincasing 21 through bolt openings 15. Since these bolts are horizontalwith respect to the HST as it is in use, the design uses stops 45 and 45b on center section 12 to contact main casing 21. Motor running surface12 is formed as an integral part of center section 10 and includes sides12 a shaped so as to fit in a bore in main casing 21 in such a manner asto allow free communication of the hydraulic oil between the areasurrounding the motor and the internal sump formed by the housingsections. The interaction of side 12 a and stops 45 a and 45 b of centersection 10 with main casing 21 supports the center section in thevertical direction and prevents rotation of center section 10 caused bytorque in the system.

The hydraulic circuit is integrally formed as porting 25 in centersection 10, although other alternative embodiments could be used. Such ahydraulic circuit generally has a high pressure side and a low pressure,or vacuum, side. Arcuate ports 13 a and 13 b are formed in motor runningsurface 12 and arcuate ports 13 c and 13 d are formed in pump runningsurface 14, and each such port corresponds to either the high pressureor low pressure sides of the hydraulic circuit. Check openings 16 a and16 b are formed in center section 10 and are similarly correlated to therespective sides of the circuit. As shown most clearly in FIG. 4, checkplugs 18 are threaded into the check openings 16, or may be fittedtherein through other methods, and act to secure check balls 20. Theoperation of check plug systems is generally known in the art and isdisclosed in U.S. Pat. No. 5,546,752, which is incorporated herein inits entirety. Check openings 16 are formed on what is generally referredto, for ease of reference, as the bottom of center section 10 althoughit is understood that the orientation is not so limited.

As shown in FIG. 2, a feature of this invention is the use of a separatefilter housing 30, which is mounted adjacent to check plugs 18 at thebottom of center section 10. In the preferred embodiment, filter housing30 is secured to the bottom of the center section 10 by washers 33 whencheck plugs 18 are screwed into openings 16, and o-rings 36 are used toassist in securing check plug 18 and to create a seal. Filter 34, whichpreferably is a 100 mesh filter, can be secured to filter housing 30using flexible plastic snaps 35 which are integrally formed with filterhousing 30. Snaps 35 then extend through corresponding openings 37formed on filter 34. This allows filter 34 to be connected to housing 30without the use of separate fasteners to minimize cost and assemblytime. Other known methods of connecting filter 34 to housing 30, such asuse of fasteners or tabs formed on filter 34, could also be used. It isalso understood that the bypass mechanism disclosed herein is notspecifically limited to the shape or design of the center section orcheck plug mechanisms disclosed, but could also be easily used withother center section or check plug designs, or even with units which donot use a center section, but have the porting mounted elsewhere in theunit such as integrally formed with the housing.

As shown in FIGS. 3 and 4, check balls 20 are mounted in internalchambers 19 of check plugs 18. A seat is formed with openings 38 so thatwhen a ball 20 is seated, no fluid can pass through opening 38. Bypassactuator 40 is mounted through use of spring 41 on tab 42 of filterhousing 30, and use of guide pins 44 on filter 34. Projections 46 areformed with actuator 40 to contact balls 20 when actuator 40 is forcedin that direction. Check plugs 18 may include bleeds 48 to allowdischarge of fluid under high pressure. Bleeds are generally known inthe art and provide a smoother transition when starting the vehicle orchanging direction, e.g., from forward to reverse, and can also providecooling for the hydraulic circuit.

In the preferred embodiment, shield 50 is secured through use of guidepins 44 and bypass actuator 40 and is positioned to block the highpressure flow of fluid from bleeds 48, in order to prevent the highpressure flow from contacting and damaging mesh filter 34. Flange 47 isformed on shield 50 for the purpose of providing additional bendingstrength to the member. Other methods of strengthening shield 50 couldalso be used. A shield mechanism could take different shapes and couldalso be integrally formed as part of actuator 40 and/or projections 46.

Actuation tab 51 is formed on actuator 40 and extends through an openingin filter 34 to contact paddle 53 of bypass actuator rod 52, which actsas a cam. Spring 41 acts to hold actuator 40 and projections 46 in the“disengaged” position shown most clearly in FIG. 4. When rod 52 isrotated, paddle 53 engages tab 51 and forces actuator 50 away fromfilter 34 and in a direction towards check balls 20, overcoming the biasforce of spring 41. In this fully engaged position, projections 46engage check balls 20 to push them off the seats and into internalchamber 19 to allow discharge of fluid from check plugs 18, thus placingthe unit in bypass.

Rotation of rod 52 back to its original position will take paddle 53 offof tab 51, and the bias force of spring 41 will force actuator 40 offballs 20 to take the unit out of the bypass mode. Bypass rod 52 isrotated by means of an external arm 54, as shown in FIGS. 6, 8 and 9 orit may be activated by other methods as known in the art. External arm54 contacts tapered flat 91 formed on rod 52 and may be secured by meansof a push-on nut. End 92 of rod 52 may rest in the housing or could beotherwise secured for rotation. End 92 of bypass rod 52 can rest in aslot 89 formed in main casing 21, as shown most clearly in FIG. 9, wherethe other elements of the bypass have been removed, in order tofacilitate ease of manufacture. As shown in FIG. 8, rod 52 is held inslot 89 by the lower side surface 12 a of motor running surface 12. Asan alternative embodiment, the diameter of motor running surface 12could be increased, and the rear side of surface 12 could have a boreformed in it or otherwise have an opening created to hold end 92 of rod52.

Other methods of actuating the bypass could also be used, such as amember extending directly through the bottom of casing 21, which coulddirectly engage tab 51. Magnet 55 can optionally be secured on rod 52 bymeans of a tab, for example. This magnet functions as a washer to assistin maintaining rod 52 in the housing, while also acting to filter loosemetal parts from the hydraulic fluid. It is understood that such abypass design could be used with a variety of hydrostatic transmissiondesigns.

As shown in FIGS. 1, 3, 5 and 6, the transmission housing includes maincasing 21 and side casing 22, which are secured by bolts 31 along avertical flange 61 defining a split line. The benefit of thisarrangement is shown most clearly in FIG. 21, where the arrangement ofmain casing 21 and side casing 22 allows for a central location of inputshaft 24 so that it can engage the driving linkage (not shown) withoutany modification of the tractor design, while still allowing use of arear discharge chute 99.

Input shaft 24 is powered by an external motive force (not shown) topower hydrostatic pump 11. Input shaft 24 extends through an openingformed in casing 21, and is supported therein by ball bearing 101. Seal122 and retaining ring 123 act to prevent leakage. Shaft 24 also extendsthrough swash plate 23 and swash plate thrust bearing 29.

As shown in FIG. 9, motor shaft 66 is drivingly engaged to gear 67,which in turn is engaged to gear 69. Gears 67 and 69 are mountedentirely within main casing 21. Gear 71 is rotatably mounted onintermediate (or jack) shaft 70. Gear 69 includes gear teeth on itsinternal diameter sized to correspond with the teeth of gear 71, suchthat gear 71 fits inside and drives gear 69. Gear 71 is also engaged todifferential bull (or spur) gear 72. A cross shaft 74 is mounted in bullgear 72 and has a pair of planet bevel gears 75 mounted thereon. Gear 71and bull gear 72 are mounted such that the plane of flange 61, i.e. theparting line between the two housing casings 21 and 22, passestherethrough. As shown in FIG. 6, axle bevel gears 77 are engaged toaxles 90A and B and to the differential.

One of the benefits of the current design is that it provides asignificantly smaller external housing for an HST than is generallyprovided by the prior art designs. As shown most clearly in FIGS. 1, 5and 8, the external housing is shaped to conform to the shape of theinternal IHT components. This minimizes the amount of material needed,which reduces cost and weight. Such a design does present potentialconcerns for strength due to the smaller amount of material used.Therefore, a plurality of support ribs 104, including flying rib 105,are formed on the external surfaces of casings 21 and 22 to provideadditional support for the housing.

As another embodiment, the housing could be constructed without theflying ribs as shown in FIG. 23, where main casing 221 and side casing222 are formed without the ribs, and axles 290A and 290B extend from thecasings 221 and 222. The internal configuration of such a unit could besubstantially the same as that shown in other embodiments herein, andinput shaft 224 could be used to drive a pump in the manner describedabove. In such an embodiment the die cast aluminum of the housing wouldnecessarily be enhanced in certain areas to increase the strength of theunit. This embodiment would improve the cooling of the unit, as air flowis maximized over the primary heat generating surfaces.

A further embodiment is shown in FIG. 24, where the housing consistingof main casing 321 and side casing 322 have been further reduced insize, so that axles 390A and 390B are rotatably supported therein butsignificant portions of said axles extend outside of the casings and aresupported at the ends thereof by bearing pillow blocks 300A and 300B.The bearing pillow blocks 300A and 300B would then be mounted to theframe of the vehicle.

Axles 90A and 90B extend from their respective housings. As shown inFIG. 10, which shows the differential with the bevel gears removed,lobed bearings 78 act to secure bevel gears 77 and axles 90A and 90B,while solid bearings 79 provide support at the ends of the axles. Theuse of lobed bearings 78 allows transfer of hydraulic oil from the maincasing to the internal chambers 88 a and 88 b of the axle horns, and thebearings include a clocking mechanism 80 to prevent rotation of thebearings 78 and the wear inherent in such rotation.

As shown in FIGS. 3, 9 and 11, pump 11 is rotatably mounted on centersection 10. Hydrostatic transmissions in the past have generally usedcradle mounted swash plates mounted directly on the housing. In thepreferred embodiment of the present invention, the speed and directionof the hydrostatic transmission may be changed by use of moveable swashplate 23, which is mounted on trunnions 26 a and 26 b secured to casings22 and 21, respectively. As shown also in FIGS. 8 and 18, trunnion 26 aincludes a step 93 to act as an oil seal surface with trunnion seal 94of casing 22, and flats 49 extend outside casing 22 to engage controlarm 108.

Bolt 97 extends through opening 121 formed in control arm 108 and isthreaded or otherwise secured directly into trunnion 26 a. Opening 121preferably has flat sides with a radius formed to improve stability ofcontrol arm 108. In the preferred embodiment, friction bearings 130interface between main casing 21 and trunnions 26 a and 26 b. It isunderstood that trunnions 26 a and/or 26 b could also run directly onthe housing elements without the need for a friction bearing.

Center section 10, pump cylinder block 17 and motor cylinder block 27are mounted completely within the main casing 21. Swash plate 23 crossesthe parting line 61 of main casing 21 and side casing 22, with theportion of the swash plate 23 that supports the pump block 11 within themain casing 21, and trunnion 26 of swash plate 23 extends across theparting line or flange 61 to interface with side casing 22. Swash plate23 is supported by main casing 21 at one end, and by side casing 22 atthe other end.

As shown in FIGS. 3 and 11, pump cylinder block 17 includes a pluralityof pump pistons 28, which engage thrust bearing 29 mounted inside swashplate 23. Motor cylinder block 27 houses motor pistons 32, which engagea fixed angle thrust bearing 39 secured in main casing 21.

Swash plate 23 includes opening 76 formed therein for input shaft 24 toextend therethrough. As shown most clearly in FIG. 12, opening 76includes a plurality of notches 76 a formed therein to provide necessaryclearance for input shaft 24. Swash plate also is shaped to include aplurality of notches 81, which can be used for clamping swash plate 23during machining thereof. The location of notches 81 provides theoptimal clamping location to avoid flexing the material duringmachining. A further benefit of notches 81, and particularly the notchesadjacent to trunnion 26 a is to provide additional clearance inside thehousing. As shown in, for example, FIG. 9, the location of notch 81avoids contact of swash plate 23 with gear 69 during certain swashorientations.

Motor shaft 66 also crosses the parting line of main casing 21 and sidecasing 22. One end of motor shaft 66 is supported by center section 10,and the other end is supported by and extends out of side casing 22, andincludes a spline 66 a for mounting to a conventional brake mechanism.Motor shaft 66 is mounted below the running surface 14 of center section10 and parallel thereto, to reduce the height of these hydrostaticcomponents.

In the preferred embodiment, housing casings 21 and 22 include aplurality of through holes 102 formed therein to be used to secure thetransmission to a vehicle frame. These holes can be sized as needed forthe application, and the number of holes can be increased or decreased.In addition to securing the transmission to the vehicle frame throughbolt holes 102, there is a need to secure the unit against rotationcaused by the torque created by the unit. It is known to attach torquebrackets to a vehicle and to secure them in some manner to the housing.One feature of this housing design is that the bolts 31 securing maincasing 21 to side casing 22 extend all the way through both casings, asshown by way of example in FIG. 5. Bolts 31 are sized to be long enoughso that torque bracket 135 can be directly mounted on bolts 31, whichallows torque bracket 135 to be secured directly to the transmissionhousing during assembly of the transmission. This eliminates the needfor separate attachment means, such as bolt holes being formed in thehousing or stud 86 as shown in FIGS. 22 and 22-A, thus lowering themanufacturing costs. It also eliminates the need for a separate assemblystep to secure torque bracket 135 to the transmission when thetransmission is mounted on the vehicle.

An oil fill port 106 is formed in main casing 21, although it could bemounted elsewhere on the unit, and is used to fill the transmission asneeded.

A further novel feature of one embodiment of the invention is in thedesign of the external expansion tank for hydraulic fluid. As shown mostclearly in FIGS. 6, 14 and 14-A, expansion tank 56 is secured to themain casing 21 and is shaped to fit securely against main casing 21. Tab132 extends from tank 56 and is secured to housing by use of fastener133, which is preferably a screw. Because tank 56 is shaped to conformto the shape of transmission main casing 21, fastener 133 and fitting 58are sufficient to hold it to the transmission.

Tube 57, which may be composed of rubber, is inserted inside tank 56 andsecured to fitting 58 and is sized to fit as close to the bottom of tank56 as possible. Tank 56, which may be composed of high densitypolyethylene, includes projection 59 having an opening formed thereinextending therefrom and matching up to boss 73 extending from maincasing 21. Fitting 58 is mounted from the inside of main casing 21 andextends into the opening of projection 59, and o-rings 63 act to preventleakage of hydraulic fluid. Fitting 58 includes a barb-type endextending into tube 57 to provide an air-tight connection, and providesan internal passage 58 a connecting passageway 58 b to the internalvolume of the transmission. In the preferred embodiment an internal hexis used to drive fitting 58 into main casing 21. As shown in FIG. 9, athrough hole 134 is formed in main casing 21 to connect to expansiontank 26 and fitting 58 is threaded therein. In the preferred embodiment,through hole 134 should be mounted as high in the unit as possible tomaximize oil fill capacity and allow for the siphoning action of tube57. Having the tube at the highest point is also preferred to preventexcessive drainage of oil from the sump in the event an air leakdevelops.

Air vent 62 is formed in the tank 56 and is covered by cap 65. A uniquefeature is the use of an additional flexible cap 64 which acts toprevent water and other foreign contaminants from entering the tank 56during operation or cleaning of the vehicle. Flexible cap 64 is shapedto conform to the external configuration of tank 56 and cover cap 65 inits entirety. The use of a flexible material such as nitrile for cap 64forms enough contact with the external housing to prevent water fromentering the system; in a preferred embodiment a small groove may beformed in cap 64 to allow improved air ventilation but still keep thesystem essentially water-tight.

During use of the hydrostatic transmission, as the hydraulic oil expandsthrough heating it will flow through fitting 58 into tube 57 and thusinto tank 56. As the oil cools and contracts, it will be drawn back inthe reverse flow from tank 56 into the main housing. The placement ofthe open end of tube 57 adjacent the bottom of tank 56 prevents thehydraulic fluid from exiting the air vent 62 at the top of the tankregardless of the orientation of the unit during operation, thuseliminating the leakage problems inherent in other prior external tankdesigns.

As shown in FIGS. 1 and 1-A, axles 90A and 90B extend outwardly fromaxle housings 21 and 22 respectively. Vehicle wheels (not shown) may besecured to each of said axles 90A and 90B through standard means such asa retaining ring (not shown) at the ends thereof, and as discussedabove, washers (not shown) may be mounted between the wheel and thehousing. In order to prevent contact of the wheels or the washers withtransmission casings 21 and 22, the present invention discloses use of aclip 82 to be secured on either end of the transmission. Clip 82 ispreferably composed of spring steel, although other materials may beused, and such a clip could be used on any type of axle housing toprevent contact between such a housing and vehicle wheels.

FIG. 1 shows the transaxle with both clips 82 in place. Clip 82 can besecured to main casing 21 and side casing 22 through use of guide pin84, which can be integrally formed with the housing as cast, in thepreferred embodiment, or can be separate members secured to the housingin known manners. Guide pin 84 engages slot 85 in clip 82 to assist ineasily locating and mounting clip 82. Clip 82 could also be securedthrough other methods known in the art and still accomplish the samefunctional benefits. Pads 83 may also be formed on main casing 21, ascast, in order to prevent rotation of clip 82 under torque, to protectpin 84 from damage. This allows for a clip 82 having a generally squareor rectangular shape, as depicted, to keep costs lower. Other methods ofpreventing rotation of clip 82 could also be used, such as shaping clip82 to fit the housing thrust surface 97. The curvature of clip 82 asshown in FIG. 15 aids in assembly of clip 82 to casings 21 and 22.

As shown in FIGS. 18, 19, 20, 22 and 22-A, an optional friction packfeature of the present invention includes a control arm 108 having anarcuate slot 110 formed therein. Carriage bolt 111 extends througharcuate slot 110 and engages nut 112, and is not secured to side casing22. Friction packs 114 a and 114 b are mounted on bolt 111 and engagecontrol arm 108. Packs 114 a and 114 b can be manufactured from agenerally flexible material such as acetal and washer 115 acts tomaintain rigidity against packs 114 a. Spacer 116, spring 117 and washer118 are also mounted on one end of the bolt 111 to maintain the properlevel of friction.

Drag link stud 120 is threaded directly into side casing 22, and extendsthrough openings in drag link 119, friction packs 114 a and 114 b andwasher 115 as well as the arcuate slot 110 in control arm 108. Arcuateslot 110 acts as an external means for limiting the movement of controlarm 108 to limit movement of the internal trunnion mounted swash plate.

The entire assembly can thus move within arcuate slot 110 on stud 120.Opening 113 can be used to attach control arm 108 to external linkages(not shown) of the vehicle. FIGS. 22 and 22-A show different embodimentsof the external linkages of the transmission, including for example thefriction pack.

A further embodiment of the differential including differential block 95is shown in FIG. 9, where like numerals designate like elements. As isknown in the art, bull or spur gears such as gear 72 used indifferentials must be properly positioned and must be of sufficientstrength to withstand the inherent forces. One method known in the artis to maintain the bull gear as a generally solid piece with openingsformed therein as needed. However, such a gear is undesirable as it addsto the weight of the unit and the manufacture of such a solid gear as apowdered metal part requires a significantly larger press machine, thusincreasing manufacturing costs. The use of block 95 allows the use of abull gear 72 having fairly large opening 96 therein to reduce the amountof material. Block 95 is held in slots 98 formed in bull gear 72 andacts to position bull gear 72. FIG. 10-A shows a more standardarrangement of a differential block 107 in the transmission.

Another optional feature of the invention is the use of an externalmeans for recording information directly on the housing in aninexpensive and durable manner. As shown in FIGS. 5 and 22, informationpads 140 consist of a series of projections formed on main casing 21 andside casing 22. The location of such a pad 140 is not critical, and pad140 could also be formed on only one of the casings 21 and 22 instead ofboth. In the preferred embodiment, eight individual units are formed,and during the machining process, one or more of these individual unitsmay be machined to encode any information the manufacturer wishes toinclude through the pattern of machined and un-machined projections. Theuse of eight individual units obviously offers a large number of codingpossibilities, and the number of projections may be increased ordecreased as needed.

As shown in FIG. 8, motor shaft 66 extends out of side casing 22, and issupported therein by friction bearings 141. Spline 66 a engages brakedisk 142. Brake arm 144 is retained by castle nut 146 and bias isprovided by spring 148. As is known in the art, movement of brake arm144 will cause yoke 149 to engage disk 142, inhibiting the rotationthereof and thereby inhibiting the rotation of motor shaft 66, slowingthe vehicle. There is also a “return to neutral” feature disclosed incertain of the embodiments of the invention. FIGS. 1 and 22 show returnarm 150 which engages ball bearing 152. Adjusting puck 154 is secured atthe base of return arm 150.

It is to be understood that the above description of the inventionshould not be used to limit the invention, as other embodiments and usesof the various features of this invention will be obvious to one skilledin the art. This invention should be read as limited by the scope of itsclaims only.

We claim:
 1. A hydrostatic transmission, comprising a casing forming asump for hydraulic fluid; a center section mounted in said casing andhaving hydraulic porting; a hydraulic pump mounted to said centersection; a hydraulic motor mounted to said center section andoperatively connected to said pump through said hydraulic porting; aplurality of check valves operatively connected to said center sectionto enable hydraulic fluid flow between said sump and said hydraulicporting; and a filter housing containing a filter and mounted in saidcasing, said filter housing secured to said center section by said checkvalves.
 2. A hydrostatic transmission as set forth in claim 1, whereinsaid hydraulic pump is an axial piston pump.
 3. A hydrostatictransmission as set forth in claim 1, wherein said hydrostatic motor isan axial piston motor.
 4. A hydrostatic transmission as set forth inclaim 1, wherein said motor is mounted generally perpendicular to saidpump.
 5. A hydrostatic transmission, comprising a casing forming a sumpfor hydraulic fluid; a hydraulic pump mounted in said casing; ahydraulic motor mounted in said casing and operatively connected to saidhydraulic pump through hydraulic porting, said hydraulic porting havingat least one check valve operatively connected to said hydraulic portingto enable fluid flow between said sump and said hydraulic porting, andat least one high pressure bleed to permit fluid flow from saidhydraulic porting to said sump; a filter mounted in said casing adjacentto said check valve and said high pressure bleed; and a shield formedbetween said high pressure bleed and said filter to prevent hydraulicfluid flowing from said high pressure bleed from contacting said filter.6. A hydrostatic transmission as set forth in claim 5, furthercomprising a filter housing mounted in said casing, with said filterbeing mounted in said filter housing.
 7. A hydrostatic transmission asset forth in claim 6, further comprising a plurality of plastic tabsformed on the external edge of said filter, and a lip formed on saidfilter housing for receiving said tabs to attach said filter to saidfilter housing.
 8. A hydrostatic transmission as set forth in claim 5,further comprising a center section mounted in said casing andcontaining said hydraulic porting and a plurality of check valves,wherein said hydraulic pump and hydraulic motor are rotatably mounted onsaid center section, and said plurality of check valves are secured tosaid center section.
 9. A hydraulic transmission as set forth in claim8, wherein each of said plurality of check valves has a high pressurebleed formed therein.
 10. A hydrostatic transmission as set forth inclaim 8, wherein said filter housing is secured to said center sectionby said plurality of check valves.
 11. A hydrostatic transmission as setforth in claim 8, wherein said hydraulic pump is an axial piston pump.12. A hydrostatic transmission as set forth in claim 8, wherein saidhydrostatic motor is an axial piston motor.
 13. A hydrostatictransmission as set forth in claim 8, wherein said hydraulic motor ismounted generally perpendicular to said hydraulic pump.
 14. Ahydrostatic transmission comprising a casing forming a sump forhydraulic fluid; a hydraulic circuit located in said casing; at leastone check valve operatively connected to said hydraulic circuit toenable hydraulic fluid flow between said hydraulic circuit and saidsump; a high pressure bleed located to permit flow of hydraulic fluidfrom said hydraulic circuit to said sump; a filter mounted adjacent tosaid check valve and said high pressure bleed, to filter hydraulic fluidflowing from said sump into said circuit; and a shield formed betweensaid bleed and said filter to prevent oil flowing from said bleed fromcontacting said filter.
 15. A hydrostatic transmission as set forth inclaim 14, wherein said hydraulic circuit operatively connects an axialpiston pump and an axial piston motor, and said pump and motor aremounted in said casing.
 16. A hydraulic transmission as set forth inclaim 14, wherein said bleed is formed in said check valves.
 17. Ahydrostatic transmission, comprising a casing forming a sump forhydraulic fluid; a hydraulic pump mounted in said casing; a hydraulicmotor mounted in said casing and operatively connected to said hydraulicpump through hydraulic porting; a plurality of check valves operativelyconnected to said hydraulic porting to enable fluid flow between saidsump and said porting; a filter mounted in said casing adjacent to saidcheck valves, said filter having locating pins integrally formedthereon; and an actuation member mounted on said locating pins of saidfilter, said actuation member positioned to engage and open said checkvalves.
 18. A hydrostatic transmission as set forth in claim 17, furthercomprising a center section mounted in said casing, and said hydraulicpump and hydraulic motor are mounted on said center section.
 19. Ahydrostatic transmission as set forth in claim 18, wherein saidhydraulic porting is integrally formed in said center section.
 20. Ahydrostatic transmission as set forth in claim 18, wherein the inputshaft for said hydraulic pump is generally perpendicular to the outputshaft of said hydraulic motor.
 21. A hydrostatic transmission as setforth in claim 18, wherein said filter is mounted in a filter housingsecured to said center section by said check valves.
 22. A hydrostatictransmission as set forth in claim 17, wherein said hydraulic pump is anaxial piston pump.
 23. A hydrostatic transmission as set forth in claim17, wherein said hydrostatic motor is an axial piston motor.